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1% clipping too high?

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    #46
    Originally posted by gadfly56 View Post

    Yes, but measuring the area under the curves is the "how to". Without the "how to," just saying "measure it" isn't very helpful.
    Most of us are just going to use software that produces estimates of energy produced and energy clipped.

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      #47
      Originally posted by gadfly56 View Post

      Yes, but measuring the area under the curves is the "how to". Without the "how to," just saying "measure it" isn't very helpful.
      Just saying "measure the area under the curve" isn't all that helpful, either; how do you do that? I would hazard a guess that most of us in here are smart enough to figure out a way to run the numbers. Duh.

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        #48
        Originally posted by 310 BLAZE IT View Post
        often you have a higher overall system efficiency when you increase clipping because the inverter operates in a more efficient region of its efficiency curve. This does not always occur but is something to look for. Where interconnection costs are a considerable effort risk, and expense, it is often more economical to oversize the system such that there is 3-4 percent clipping.
        Indeed. I'm currently trying to decide between a 3.8kW and 5.0kW inverter (SMA SunnyBoy line because of "secure power supply") for a system that is panel-constrained to 4.86kW under STC conditions. No-brainer to pick the 5.0kW inverter, especially since it's only $70 more (out of $1000 or so) ? Maybe not. Look at the efficiency curve:

        Click image for larger version

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        Once you get below 10% or so of maximum output, the efficiency is going to roll off quite a bit; and you're gonna take that hit more with the larger capacity inverter. That could hurt your production a fair amount if the panels are operating at a low output a good bit of the time.

        Meanwhile, clipping using the 3.8kW model is not as bad as you'd think, because those panels are never going to actually generate 4.86kW of DC (because that's under un-realistic testing conditions).

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          #49
          Originally posted by ggunn View Post
          Just saying "measure the area under the curve" isn't all that helpful, either; how do you do that?
          Seem to be some newbies here - as am I, maybe just a wee bit further up the learning curve than one or two others here. So, if you don't know about the PVWatts calculator (https://pvwatts.nrel.gov/pvwatts.php) please do check into it. Among other things, it will tell you the DC and AC production for every hour of every day of the year. A little Excel-fu, and you've got the area under the curve.

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            #50
            Originally posted by RustyShackleford View Post
            Seem to be some newbies here - as am I, maybe just a wee bit further up the learning curve than one or two others here. So, if you don't know about the PVWatts calculator (https://pvwatts.nrel.gov/pvwatts.php) please do check into it. Among other things, it will tell you the DC and AC production for every hour of every day of the year. A little Excel-fu, and you've got the area under the curve.
            I was being just a bit facetious; sorry.

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              #51
              Originally posted by RustyShackleford View Post

              Indeed. I'm currently trying to decide between a 3.8kW and 5.0kW inverter (SMA SunnyBoy line because of "secure power supply") for a system that is panel-constrained to 4.86kW under STC conditions. No-brainer to pick the 5.0kW inverter, especially since it's only $70 more (out of $1000 or so) ? Maybe not. Look at the efficiency curve:

              Click image for larger version

Name:	Screen Shot 2019-11-12 at 8.09.23 PM.png
Views:	53
Size:	99.0 KB
ID:	2544164

              Once you get below 10% or so of maximum output, the efficiency is going to roll off quite a bit; and you're gonna take that hit more with the larger capacity inverter. That could hurt your production a fair amount if the panels are operating at a low output a good bit of the time.

              Meanwhile, clipping using the 3.8kW model is not as bad as you'd think, because those panels are never going to actually generate 4.86kW of DC (because that's under un-realistic testing conditions).
              I think the effect of running at lower efficiency with a larger inverter will be very small. Just for rough numbers, and assuming the efficiency curves for the two inverters are similar (don know if that is true). : Say for round numbers you are considering a 3KW and a 6KW inverter. Say you are running 300 watts which is 10% of the 3KW and 5% of the 6KW. That is 95% vs 91% respectively, or 285 vs 273 watts. I think output within that 300 watt range is a rather small portion of the overall energy portfolio. I would certainly make up for a little bit of clipping, but Im not sure any substantial amount.
              Ethan Brush - East West Electric. NY, WA. MA

              "You can't generalize"

              Comment


                #52
                Originally posted by RustyShackleford View Post

                Indeed. I'm currently trying to decide between a 3.8kW and 5.0kW inverter (SMA SunnyBoy line because of "secure power supply") for a system that is panel-constrained to 4.86kW under STC conditions. No-brainer to pick the 5.0kW inverter, especially since it's only $70 more (out of $1000 or so) ? Maybe not. Look at the efficiency curve:

                Click image for larger version

Name:	Screen Shot 2019-11-12 at 8.09.23 PM.png
Views:	53
Size:	99.0 KB
ID:	2544164

                Once you get below 10% or so of maximum output, the efficiency is going to roll off quite a bit; and you're gonna take that hit more with the larger capacity inverter. That could hurt your production a fair amount if the panels are operating at a low output a good bit of the time.

                Meanwhile, clipping using the 3.8kW model is not as bad as you'd think, because those panels are never going to actually generate 4.86kW of DC (because that's under un-realistic testing conditions).
                I think the effect of running at lower efficiency with a larger inverter will be very small. Just for rough numbers, and assuming the efficiency curves for the two inverters are similar (don know if that is true). : Say for round numbers you are considering a 3KW and a 6KW inverter. Say you are running 300 watts which is 10% of the 3KW and 5% of the 6KW. That is 95% vs 91% respectively, or 285 vs 273 watts. I think output within that 300 watt range is a rather small portion of the overall energy portfolio. I would certainly make up for a little bit of clipping, but Im not sure any substantial amount.
                Ethan Brush - East West Electric. NY, WA. MA

                "You can't generalize"

                Comment


                  #53
                  Originally posted by electrofelon View Post
                  I think the effect of running at lower efficiency with a larger inverter will be very small.
                  I think you're right. I have been going nuts with the hourly output file from PVWatts, and tried to make my own computation of AC power, using the DC power number it gives, and a piecewise-linear approximation to the efficiency curve I posted above. And it does seem to make well under 1% of difference.

                  Last edited by RustyShackleford; 11-13-19, 11:42 PM.

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